Iceland is a part of the Schengen area. Japanese nationals do not need a visa for a stay of up to three months in all within the Schengen area. The total stay within the Schengen Area must not exceed three months in any period of six months.
The validity of the passport or recognized ID card must be at least three months beyond the proposed stay.
For further information, see here: http://www.utl.is/index.php/en/who-does-not-need-a-visa#J